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Designs that vary the bias against the musical signal will generally have bias currents at or
below the signal level.  This is certainly an improvement from the viewpoint of energy
efficiency, but the sound reflects the lesser bias point.

I authored the first patent on the dynamically biased Class A amplifier in 1974, however I have
not used the technique for the last 15 years.  The reason is that I found the quality of sound
associated with an efficient Class A operating mode inferior in depth and less liquid at high
frequencies, simply because it operates at reduced bias at low levels.  Given the plethora of
cool running “Class A” amplifiers on the market, you might say I opened a Pandora’s box.

A very important consideration in attempting to create an amplifier with a natural characteristic
is the selection of the gain devices.  A single ended Class A topology is appropriate, and we
want a characteristic where the positive amplitude is very, very slightly greater than the
negative.  For a current gain device, that would mean gain that smoothly increases with
current, and for a tube or field effect device a transconductance that smoothly increases with
current.

Triodes and Mosfets share a useful characteristic:  their transconductance tends to increase
with current.  Bipolar power devices  have a slight gain increase until they hit about an amp or
so, and then they decline at higher currents.  In general the use of bipolar in a single ended
Class A circuit is a poor fit.

Another performance advantage shared by Tubes and Fets is the high performance they
deliver in simple Class A circuits.  Bipolar designs on the market have between five and seven
gain stages associated with the signal path, but with tubes and Mosfets good objective
specifications are achievable with only 2 or 3 gain devices in the signal path.

Readers of The Audio Amateur Magazine will be familiar with my “Zen” design, which uses a
Mosfet in a power amplifier which has only one gain stage, and only one gain transistor.
Research continues at Pass Labs in improving the performance of very simple gain circuits.

Yet a third advantage tubes and Mosfets have over bipolar devices is their greater reliability at
higher temperatures.  As noted, single ended power amplifiers dissipate comparatively high
wattages and run hot.  Bipolar devices are much more prone to failure at high temperatures.

In a decision between Triodes and Mosfets, the Mosfet's advantage is in naturally operating at
the voltages and currents we want to deliver to a loudspeaker.  Efforts to create a direct
coupled single ended triode power amplifier have been severely limited by the high voltages
and low plate currents that are the province of tubes.  The commercial offerings have not
exceeded 8 watts or so, in spite of hundreds of dissipated watts.

Transformer coupled single ended triode amplifiers are the alternative, using very large
gapped-core transformers to avoid core saturation from the high DC current, but they suffer
the characteristic of such a loosely coupled transformer as well.
The promise of the transconductance characteristic in power amplifiers in providing the most
realistic amplified representation of music is best fulfilled in Mosfet single ended Class A
circuitry where it can be used very simply and biased very high.

Summary of Contents for Aleph 1.2

Page 1: ...Pass Laboratories Aleph 1 2 Owner s Manual Serial ...

Page 2: ...nts of loudspeakers Previous methods of loading the output stage have used networks consisting of resistors coils transformers and active current sources all of which offer an optimal load line based on a resistive load The Aleph 1 2 has a current source topology which optimizes performance for a wide range of impedance and reactance in the load improving all aspects of performance into real louds...

Page 3: ...rom the wall at all times and during idle operation nearly all of this energy will appear as heat on the heat sinks Good ventilation is vital to the proper operation of the amplifier It has been adjusted for optimal performance at room temperature but will work well between 50 and 90 degrees Fahrenheit 10 to 33 Celsius You should leave at least 12 inches clearance on the sides and top The amplifie...

Page 4: ...ts are not equal and products with higher power wider bandwidth and lower distortion do not necessarily sound better Historically that amplifier offering the most power or the lowest IM distortion or the lowest THD or the highest slew rate or the lowest noise has not become a classic or even been more than a modest success For a long time there has been faith in the technical community that eventu...

Page 5: ...rtue of only changing the bias and the expectations of the experimenter As the bias is reduced the perception of stage depth and ambiance will generally decrease This perception of depth is influenced by the raw quantity of bias current If you continue to increase the bias current far beyond the operating point it appears that improvements are made with bias currents which are much greater than th...

Page 6: ...than the negative From this we see that air is phase sensitive As a result of its single ended nature the harmonic content of air is primarily 2nd order that is to say most of the distortion of a single tone is second harmonic The phase of this distortion reflects the higher positive pressure over the negative Air s transfer curve also shows also that it is monotonic which is to say its distortion...

Page 7: ...ain the degree of control and precision required to produce the range and subtlety required by music And so it is with single ended amplifiers Only one linear circuit topology delivers the appropriate characteristic and that is the single ended amplifier Single ended amplification only comes in pure Class A and is the least efficient form of power stage you can reasonably create typically idling a...

Page 8: ...performance they deliver in simple Class A circuits Bipolar designs on the market have between five and seven gain stages associated with the signal path but with tubes and Mosfets good objective specifications are achievable with only 2 or 3 gain devices in the signal path Readers of The Audio Amateur Magazine will be familiar with my Zen design which uses a Mosfet in a power amplifier which has ...

Page 9: ...imple topology which is a key part of the sound quality Other solid state amplifier designs have five to seven gain stages in the signal path in order to get enough gain to use feedback to provide adequate performance In this amplifier we get greater linearity by providing much more bias through two gain stages a differential input stage and the output transistors Mosfets provide the widest bandwi...

Page 10: ...and simple Mosfet designs It becomes more important with complex circuit topologies where there is heavy dependence on feedback correction but even then its importance has been overstated The amplifier is powered by a toroidal transformer which charges 10 Farad capacitance This unregulated supply feeds the output transistors only with a full power ripple of less than a volt The power draw of this ...

Page 11: ...00 watts 8 ohms 3 300 watts 4 ohms Input Impedance 10 Kohm unbalanced 25 Kohm balanced differential Damping factor 70 nominal Output Noise 800 uV unweighted DC offset 100 mv Power Consumption 500 watts Temperature 25 degrees C above ambient Dimensions 16 5 W x 16 5 D x 10 5 H Shipping Weight 120 lb PASS Pass Laboratories PO Box 219 24449 Foresthill Rd Foresthill CA 95631 tel 916 367 3690 fax 916 3...

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